JPQL join 2表:编写jpql

Question

我有2个表第一个表 - 事件第二个表 - 类别

@Entity
public class Event {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long eventId;
private String name;
@ManyToOne
@JoinColumn(name = "category_id")
private EventCategory category;
//getters and setters
}

@Entity
@Table(uniqueConstraints = @UniqueConstraint(columnNames = { "CATEGORY" }))
public class EventCategory {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long categoryId;
@NotNull
private String category;
@OneToMany(mappedBy = "category", cascade = CascadeType.ALL)
private List<Event> events;
}

现在我想要所有类别都有价值的事件.我对jpa很新.很难为此编写查询.如果你可以帮助我,那将会很有帮助.

编辑:我在事件表中存储categoryId.我希望能够按类别名称搜索.类别名称仅保存在类别表中.所以我想我需要与类别表进行联接.

Answer 1

使用这样的查询:

final String query = "SELECT e FROM Event e WHERE e.category = :category";

您可以使用多种方法检索记录,但这里有一个:

final EventCategory category = ...;
TypedQuery<Event> query = entityManager.createQuery(query, Event.class);
query.setParameter("category", category);
final List<Event> events = query.getResultList();

*编辑*

默认情况下,上面说明的方法将根据为类别对象定义的主键进行搜索.如果要搜索其他属性,则需要在JPQL中指定:

final String query = "SELECT e FROM Event e WHERE e.category.name = :categoryName";

final String categoryName = ...;
TypedQuery<Event> query = entityManager.createQuery(query, Event.class);
query.setParameter("categoryName", categoryName);
final List<Event> events = query.getResultList();